Are nesting wading birds killing Cork Oak trees?

My Dad mentioned an article he had read recently about birds killing off Cork Oak trees and I meant to research it earlier, but forgot about it. I don’t know what triggered the recollection this morning, but whatever it was led me to find this article on what seems to be an interesting website, new to me, that is produced by the Natural Research Environment Council and titled Planet Earth. The link is http://planetearth.nerc.ac.uk and they invite comments on their articles if you think you know better! It’s an interesting article, although it does make it sound as though all the regions’ Cork Oaks are under this threat, rather than those growing in the immediate Doñana area.

Bird conservation leads to tree death

14 December 2010, by Tamera Jones

Conservationists have been so successful at protecting endangered birds in a Spanish nature reserve, that the birds are now killing the reserve’s ancient cork oak forest, say scientists. This may mean some colonies will have to be moved to protect the trees, some of which date back to the seventeenth century.

The trees are the last part of a huge cork oak forest which once covered south-western Spain, and provide food for deer, wild boar and small mammals. The endangered Iberian Lynx often also uses old cork oak trees as breeding dens.

Damaged tree (image: Maria T.Dominguez)

Over the last 40 years scientists had noticed that trees used by wading birds during the nesting season seemed to die more often than trees not used by birds in the Doñana Biological Reserve – a World Heritage site – in south-western Spain. But they couldn’t be sure if firstly the damage could be blamed on birds, and secondly how the birds were damaging the trees. So Dr Luis García from the Instituto de Recursos Naturales y Agrobiología de Sevilla (CSIC), other Spanish scientists and a researcher from the Centre for Ecology & Hydrology decided to find out.

In reserves where bird populations are protected, tree damage can often be blamed on bird populations that have got too big. So Garcia and his team reasoned this could well be happening in the Doñana Biological Reserve.The team analysed the health of 60 ancient cork oak trees, sampled leaves and soil, and worked out how many leaves were covered with poo. They write that, ‘an intense and persistent occupation of trees by colonial wading birds induces large changes in the soil composition and contributes to the decline of the centenarian cork oaks.’

They found that salts in the soil came from the birds’ excrement. Not only does excess salt stunt the trees’ growth, eventually killing them, but the birds break branches, tear off twigs for nests and their droppings are bad for the trees’ leaves. With around 70 nests in any one tree, the researchers found that the nesting birds produce so much poo it upsets the delicate balance of the soil which nurtures the oaks.

The birds’ faeces lead to high concentrations of salts in the soil, to which cork oak trees are particularly sensitive – the salt makes it hard for the trees to absorb enough water. The researchers found that cork oaks which are home to birds like the white stork, the spoonbill and the grey heron are in much worse condition than trees with no nesting birds.

‘If populations of these birds continue to rise, the effect will increase. The source will still be there,’ says Cristina Aponte from the Instituto de Recursos Naturales y Agrobiologia de Sevilla, and co-author of the study published in Biological Conservation. ‘New trees will be used by the colony as the occupied trees die.’

This case of one conservation success leading to the decline of other endangered species isn’t as rare as you might think. Problems crop up when different species have different needs and they live in the same habitat. In Polish forests, cormorants and protected wading birds have harmed ancient trees, elephants destroy endangered plants and trees in parts of South Africa, and the rare chamois in the Pyrenees feeds on the equally endangered larkspur plant.

For managers of the reserve, there is clearly a trade-off between maintaining the area for nesting birds and preserving the ancient cork oak forests. One solution to the problem would be to relocate the colony to a region where the trees aren’t as valuable or rare. But the size of the colonies could make relocation challenging.

As the climate changes, warmer temperatures could bring further problems.

‘Increased water-stress under climate change could exacerbate the problem, because trees won’t be able to get the water they need,’ explains Aponte.

Around 95 per cent of the cork oak forest in south-western Spain was devastated by human exploitation between the seventeenth and twentieth centuries. But remaining trees were kept for producing cork, and to support cattle and game.

Now wading birds nest in the trees from February to July, with colony sizes varying from year to year from 150 to 13,000 pairs.